Bubbling-bed fluidized bed combustors are capable of burning a wide variety of fuels, but not all at once. For example, volatile fuels, such as plastics, and light-weight fuels, such as paper or refuse-derived fuel (RDF) fluff, which comes from municipal solid waste, tend to float at the surface of the combustion bed instead of mixing into it, as with ordinary fuels. This causes above-the-bed combustion that is accompanied by unacceptably high temperatures above the bed, while cooling the bed itself, perhaps below the ignition point, and in any case, reducing the combustion efficiency.
To burn such fuels within the bed, special low-density bed materials have to be used or, more commonly, the fuel is densified by such processes as pelletizing, at significant added cost.
Gaseous fuels and very fine fuels, such as sawdust and pulverized coal, also require special provisions. Unlike coarse solid fuels which are spread across the bed by its motion, gaseous and fine fuels blow straight out of the bed above the fuel injection points, forming fuel-rich flame spouts above the bed. As with the low-density fuels, the above-the-bed flames create unacceptably high temperatures above the bed and unacceptably low temperatures in the bed. The fuel-rich spouts can also contribute to air pollutant emissions and reduced combustion efficiency. As a result, gaseous and fine fuels can be efficiently burned only if the spacing between the fuel injection points is made very small, on the order of a few inches, which requires a great multiplicity of feedpoints.
The combustion of liquid fuels, such as residual oils, also requires a multiplicity of feedpoints for similar reasons. In addition, the injectors must be specially designed to prevent the dripping of the oil onto the distributor plate where it would agglomerate.
Furthermore, the size of the piping and nozzles for injecting gaseous fuels is necessarily different from that of liquid or solid fuels due to the wide differences in the densities of the fluids. Thus, separate fuel feed systems are required for each fuel, there being no possibility of a universal fuel feed system which can handle all types of fuel.
Solid fuels create additional problems. The performance of coal fired fluidized bed combustors is best when the coal is injected pneumatically at a multiplicity of under-the-bed feed pipes. But such systems require that the coal must be prepared before it can be conveyed to the combustors. Such preparation includes metering, crushing, drying, and then accurately splitting the coal flow into a number of streams, all at added cost.
Other solid fuels, such as wood chips, are generally too large to fit through the coal pipes and require yet other preparation, transmission and injection systems. Other non-solid fuels, such as sludges, slurries or sticky fuels, such as tars, also need customized fuel handling and injections systems.
As a result, bubbling-bed fluidized bed combustors tend to be designed for only a single type of fuel, or perhaps a limited number of fuels. In contrast, circulating-bed fluidized bed combustors have been able to overcome this problem by injecting any of a wide variety of fuels into a stream of circulating solids, using only a few types of injections and feedpoints.
It is the purpose of the present invention to give the bubbling-bed fluidized bed combustor the same or better capability without incurring the very tall height, and its associated cost, of the circulating-bed fluidized bed combustors. As will be seen, this is accomplished by providing the combustor with universal feed systems, including gas sparge pipes for volatiles and conveyors for coarse materials, with the pyrolzer serving as a fuel processor to provide only appropriate gaseous and coarse fuel streams.
A second limitation of bubbling-bed fluidized bed combustors relates to performance, such as air pollutant emissions and combustion efficiency. In particular, the emissions of oxides of nitrogen and carbon monoxide are not as low as those of the circulating-bed fluidized bed combustors, and combustion efficiency is generally somewhat lower.
A third limitation of coal-fired bubbling-bed fluidized bed combustors relates to their maximum feasible size. The under-the-bed coal feed pipes that are needed to maximize the performance of the bubbling-bed fluidized bed combustors are regarded as high maintenance devices because they are subject to erosion and clogging. Several hundred such pipes are required with a utility-size combustor, which is considered impractical, and the requirement for such a large number of feed pipes has inhibited the use of bubbling-bed fluidized bed combustors in the larger size.
A fourth limitation of fluidized bed combustors is their relatively low operating temperature, established by the need to avoid the slagging of the ash in the fuel. Applications exist for which it would be attractive to achieve somewhat higher temperatures than are feasible in fluidized bed combustors, while retaining the use of such fluidized bed features as "in situ" pollution control. Such applications include combined-cycle (gas turbine-steam turbine) cogeneration plants, hazardous waste incinerators and high-temperature industrial furnaces.
More particularly, as illustrated in U.S. Pat. Nos. 4,135,885, 4,279,205, 4,279,207, 4,303,023, and 4,499,857, and assigned to the Assignee hereof, all of said patents being incorporated herein by reference, multiple-bed combustors are illustrated in which coal is burned and in which certain improvements in control and efficiency are described, with the earliest of these patents, U.S. Pat. No. 4,135,885, describing a dual-bed chemical reactor for burning and desulfurizing coal that has a first upstream fluid bed which primarily burns the coal and a downstream fluid bed which desulfurizes. While these combustors work exceptionally well for use with coal, they must be substantially reconfigured with any other type of raw fuel to be used.
While the above patents summarize effective techniques for the burning of coal in fluidized bed combustors as mentioned above, there is an increased need for the combustion of other fuels, such as waste coal, petroleum coke, refuse-derived fuel (RDF), sewage sludge, wood (including sawdust), industrial waste (including plastics and paper), peat, tar, oil, gas (including low-BTU gas), shredded tires, liquates and agricultural wastes (including bagasse, rice hulls and peanut shells as examples). These fuels have BTU contents of 2,000-22,000 BTU/lb and maximum ash and moisture contents of 85%. The adaptability of fluid bed combustion furnaces to process such a variety of fuels often necessitates complete reconfiguration of the furnaces to adapt them to the particular fuel to be burned.
In the past, pyrolyzers have been used to process raw fuel to evolve gases to be used to raise the temperature of flue gases emerging from fluidized bed combustors. Such systems described in other patents assigned to the Assignee hereof which deal with coal-burning arrangements are U.S. Pat. Nos. 4,051,791 and 4,253,409, with the above patents being related to discharge of char and combustion materials in the form of gases to a fluidized bed in the form of a gaseous refined state. Note that in these patents, the pyrolzers require a separate heat source.